Many attempts have been made in the prior art to construct precision accelerometers for use in guidance and navigation systems, and which have the capability of accurately measuring accelerations of the vehicle along the three coordinate axes. The most common prior art accelerometer is the spring-suspended mass type. However, such prior art instruments are usually capable of measuring accelerations only along one or two axes. Therefore, two or more of such prior art spring-suspended accelerometers are required in order to measure accelerations along the three coordinate axes. Moreover, instability and hysteresis of the springs limit the accuracy of this type of accelerometer.
Accelerometers have been suggested in the prior art which are capable of measuring accelerations along the three coordinate axes, and which do not involve the need and constraints of mechanical springs. One such instrument is described, for example, in Browning U.S. Pat. No. 3,148,456. An accelerometer is described in the Browning patent which is capable of measuring accelerations along any of the three coordinate axes. This is achieved by measuring variations in the intensity of nuclear radiation from a radioactive spherical inertial sensing mass which is positioned within the spherical chamber. The positioning of the sensing mass within the chamber is accomplished by virtue of the static and dynamic electrostatic fields produced and maintained within the chamber by the emission and subsequent absorption of alpha and beta nuclear particles from the inertial sensing mass, the nuclear particles having their origin in radioactive sources contained within the mass.
As described in the Browning patent, by utilizing radioactive source materials located in the central inertial sensing mass, it is possible to position the central mass centrally within the spherical chamber in a condition of stable equilibrium, this being achieved by virtue of the emission and subsequent absorption of the electrically charged alpha and beta particles from the central mass within the spherical chamber, and without the need for external circuitry or external power sources to supply the required energy. The symmetrical electrostatic field normally developed within the spherical chamber in the Browning system no longer retains its symmetry when the inertial sensing mass is displaced by some acceleration, and the charged particle emission from the mass then causes a restoring force to be developed which is equal in magnitude but opposite in direction to the displacing force, so that a condition of stable equilibrium exists for the displaced mass relative to the center of the spherical chamber so long as the acceleration continues.
The accelerometer of the present invention is of the same general type as the accelerometer described in the Browning patent, in that it also includes a radioactive source positioned within an enclosed chamber. However, the accelerometer of the present invention uses a radioactive particle, instead of utilizing a sphere of substantial dimensions, as the inertial sensing mass. Also, the accelerometer of the present invention uses a cubical chamber, rather than a spherical chamber. An important feature of the accelerometer of the present invention is the provision of a feedback force which tends to return the particle to its central position in the presence of accelerations, and which, in itself, serves as a measure of the accelerations along the various coordinate axes.
The inertial sensing mass in the Browning accelerometer, as mentioned above, is formed by an inner sphere prepared with radio-nuclides which generate both alpha and beta emissions, and which is positioned in an evacuated chamber formed by a larger outer spherical casing. The outer spherical casing of the Browning accelerometer passes the beta-emitting radio-nuclides and yields three or more beta particles for each alpha particle. The inner sphere, constituting the inertial sensing mass, becomes electrostatically positively charged to a high potential during the operation of the Browning accelerometer, this potential being limited only by the leakage and recombination phenomena in the partial vacuum which is established within the spherical casing. The magnitude of the positive potential on the inner sphere is also affected by the rate of alpha and beta emissions, and is thus influenced by the depletion of the radio-nuclides with time.
The electrostatic charge on the inner sphere of the Browning accelerometer is the equivalent of the spring in the prior art spring-suspended mass accelerometers, and when the Browning instrument is subjected to an acceleration, the inner sphere will be displaced from its central position, as explained above, and the alpha and beta emissions will be re-distributed and consequently a restoring force is developed. However, the acceleration cannot be measured accurately because the Browning instrument does not incorporate a force feedback system, and the accuracy with which the acceleration can be measured is dependent, inter alia, upon the precision of the pickoff. Also, as mentioned above, the potential of the central sphere in the Browning instrument, and thus the scale factor of the instrument varies with time as the radio-nuclides are depleted, and constant recalibrations are required.
Accelerometers using radioactive inertial sensing masses, and employing force feedback systems, are known to the prior art. For example, such an accelerometer is described in Cohen U.S. Pat. No. 3,120,130. However, the instrument described in the Cohen patent is a single-axis accelerometer which uses a spring-restrained inertial sensing mass and a magnetic force-feedback system. Therefore, the Cohen instrument is limited in that it is a single axis device, and in that it requires a mechanical spring system to restrain the inertial sensing mass, so that its accuracy is limited by the instability and hysteresis of the spring. Also, the alpha-emitting nuclide used in the Cohen device has a finite life, so that the Cohen instrument, like the Browning instrument, has a scale factor which changes with time.
As mentioned above, the instrument of the present invention is similar to the Browning instrument in some respects in that it uses a radioactive inertial sensing mass centrally positioned within an outer casing, and normally held in that position by virtue of static and dynamic electrostatic fields produced and maintained within the chamber by the emission and subsequent absorption of nuclear particles from the mass.
In the instrument of the present invention, a selfcharging radioactive particle is used as the inertial sensing mass, and the particle is suspended within an evacuated cubical chamber. The particle may emit, for example, alpha-particles, beta-particles, gamma-particles, or positrons. The particle is very small, and it carries only a very limited amount of the radioactive material, the amount being insufficient properly to position the mass within the casing at deflected positions in the presence of accelerations. Instead, the system of the present invention uses an external electronic feedback circuit to provide feedback forces to the chamber, which tend to return the particle to its central position in the presence of accelerations, and the circuit also provides output signals which are a measure of the force required to return the particle to its central position, these forces being directly proportional to the accelerations being measured. The invention provides, therefore, a precise and accurate accelerometer, which is not subject to any of the limitations of the prior art accelerometers, as described above, even though a small and relatively weak radioactive particle is used. In the unit of the invention, and unlike the prior art instruments, the field emission from the radioactive particle is used to stabilize its potential, so that the potential of the particle remains constant, and the instrument is capable of precise and accurate operation over long time intervals without recalibration. The electrostatic field surrounding the particle should be homogeneous. Distortions in the field may be minimized to be as small as two parts per million by making the chamber size of the order of 1 inch and the particle diameter of the order of 0.001 - 0.002 inches. In this way the ratio between the geometrical imperfections of the spherical particle and the internal dimensions of the chamber can be reduced to be a few parts per million.